Multi junction solar cells are known to convert solar energy to electricity. Such cells have found particular utility in various terrestrial and extra-terrestrial applications. Gallium Arsenide (GaAs) has been of interest for many years as a high-efficiency material for use in the manufacture of photovoltaic cells. The bandgap for GaAs at 1.42 eV at 300K is ideal for a photovoltaic device operating in the solar spectrum. Further, solar cells based on GaAs have been found to operate well at high temperatures and low temperatures, such as those found in outer space (for example, from about −180° C. to about 150° C.). Solar cells including multi junction III-V solar cells such as the state-of-the-art GaInP/GaAs/Ge triple junction solar cells employing GaAs-containing layers are known to undergo degrading performance during exposure to charged particle radiation, such as, for example, radiation that routinely occurs in space. One reason thought to contribute to such degradation is that interstitial arsenic atoms created by interaction with charged particle impacts to the crystal lattice in the GaAs layers are relatively slow in terms of diffusing to arsenic vacancies caused by the exposure to radiation. Therefore, once such a vacancy occurs, the arsenic atom in the GaAs has a relatively low probability of returning to a lattice site. The result is that the arsenic vacancy in the lattice acts as a trap for minority carrier recombination, thus decreasing the carrier lifetime and diffusion length of the minority carriers generated in the subcell structure. As diffusion lengths decrease, the overall performance of the solar cell degrades under increased energetic particle bombardment (radiation). As the arsenic is deemed easier to remove relative to other materials in the GaInP/GaAs/Ge stack, the GaAs subcell becomes the weakest structural point in the multi-junction under irradiation. As a result, triple junction solar cells employed, such as, for example, with space satellite applications, are presently limited in their performance and longevity by the properties of the GaAs middle cell. Even solar cells using metamorphic GaInAs alloys and similar GaInAsP and GaInAsSb alloys for lower bandgap subcells have been shown to degrade following irradiation.
At the atomic level, radiation impacting a GaAs cell layer causes displacements in the lattice structure. This disruption in the lattice results in a decrease of the minority carrier's lifetime, leading to a decrease in overall cell life. However, GaAs cells have been shown to have the capacity to repair themselves, albeit at a very slow rate, after sustaining damage due to irradiation.
One known solution for countering the effects of solar cell degradation due to exposure to radiation is to design a triple junction solar cell to insure that the GaAsP top subcell limits performance prior to radiation exposure. This state is typically referred to as “beginning of life” or BOL. After radiation exposure, both the GaInP top subcell and the GaAs middle subcell reach substantially equivalent current generation performance and are said to be “current matched” at “end-of-life”, or EOL. Typically, as understood by one skilled in the art of space solar cell fabrication and design, this EOL condition is measured in equivalent 1-MeV electron fluences of between 3×1014 and 3×1015 charged particles per square centimeter as determined by the specific mission where the cells are employed. It should be noted that the terms BOL and EOL are understood by those skilled in the field to be associated with mission specific lifetime. Charged particle EOL, for example, can also be determined by proton damage. However, EOL is typically rated in equivalent damage by 1-MeV (1 mega-electron volt) energy electron damage. Such ratings are typically generated at 5e14 and 1e15 1-MeV electrons/cm2 as the majority of missions range between 1e14 and 3e15 1-MeV electrons/cm3. Additionally, cells can be engineered with ionized doping profiles to enable a field collection that does not change as a consequence of radiation, thus improving the minority carrier diffusion length regardless of irradiation.
Another known pathway employs thinner GaAs subcell layers. However, the use of thinner GaAs layers produces a trade-off in that thinner cells absorb less light/energy and are therefore inherently less efficient at “beginning-of-life”. A further known step involves adding optical elements such as, for example reflectors to enable use of a thinner GaAs cell by allowing light a second “pass” through the thin GaAs subcell.
Accordingly, both “beginning-of-life” and “end-of-life” performance solutions currently available offer limited benefit since the GaAs-based subcells are susceptible to radiation damage. In next generation nJ multijunction solar cells (where n is 3, 4, 5 . . . or more junctions), the presently known attempts that address the radiation-sensitive GaAs subcell and its subsequent degradation become even less attractive and difficult to implement, as different subcells will degrade at differing rates. Doping profiles and thinner subcells only serve to worsen the problem, as the after-effects that occur in the GaAs-based layers are managed on minority carrier properties. Such presently known attempts at offering a solution to the problem are therefore limited relative to the amount of damage to the GaAs-based subcell layers that they are able to mitigate.
Additionally, there are trade-offs for thinner cells that diminish the “beginning-of-life” efficiency. For example, additional optical reflection involves complex epitaxial structures, such as distributed Bragg reflectors (DBR), or additional, and hence, complex and costly processing to turn the epitaxial stack into a fully functional cell. In general, efficiency is derived from a product of current output (number of electrons flowing) multiplied by their energy (voltage). As the cells are thinned to improve radiation tolerance, they typically absorb less light, thus generating less current. In tandem monolithic multi junction devices, such as, for example, the 3J GaInP/GaAs/Ge, the subcells are combined in series, meaning that their individual subcell voltages “add” and the current is limited by the lowest subcell. Therefore, as the GaAs subcell is thinned, it will produce less current and thereby reduce the current of the overall stack. This causes the overall efficiency to be limited by such current. Current balancing the subcells maximizes the overall efficiency of the stack by maintaining even production of current in all three subcells. This is typically achieved by making the GaAs layer optically thick (i.e. greater than a thickness of about 3 μm).
BOL efficiency is the un-irradiated (i.e. starting, or “beginning”) efficiency of the device, such as, for example, in the case of a space vehicle, prior to launch into space). This BOL efficiency is therefore defined in testing as the starting point (zero particle fluence). Efficiency is the wall-plug efficiency or maximum electrical power output by the solar cell divided by the input power from the sun (a constant), which, in space, is taken as 1.353 W/cm2.